In a typical application of a known array antenna, a set of beams are formed to span a field of view extending to ±45° in azimuth, with each of the beams pointing at fixed scan angles. To ensure that the beams span the field, tight limits may be set on the allowable crossover levels between adjacent beams so that there are no significant gaps in the coverage of the field. Nominally, the beams would be required to intersect at or above the −3 dB points in their far-field radiation patterns at an intended frequency of operation. However, it is known that the width of beams for an array antenna is inversely proportional to the frequency of the radiation. Hence, in the particular application considered, where the beam peaks are at fixed scan angles, the crossover points of adjacent beams vary considerably according to the frequency of operation so that, at higher frequencies, gaps are likely to develop in the coverage of the intended field. This limits the range of frequencies over which a known design of co-phased array antennae may be used.
It is known to try to overcome this problem of narrowing beam widths by varying the amplitude of signals across the elements of an array antenna according to frequency of operation. In one known approach, it has been suggested that “apodising” filters be connected to each element of an array to control the amplitude of the respective signals. Apodising filters provide low attenuation at lower frequencies and high attenuation at higher frequencies. The ideal filter characteristic for each element of the array is dependent on the position of the element within the array. For elements at the center of the array the filters should have a filter characteristic that varies only slightly with frequency whereas, for elements towards the edge of the array, the filters should have a filter characteristic that varies greatly with frequency. Thus, at the lowest frequencies, the filters would provide an approximately uniform illumination across the array, leading to a relatively narrow beam for this frequency of operation. At the higher frequencies the filters would produce a highly tapered illumination through greater attenuation of signals for elements towards the edges of the array, leading to a relatively wide beam for this frequency of operation and so compensating for the natural narrowing of the beam at those higher frequencies. By synthesising the ideal distribution of signal amplitude at each frequency, a detailed apodising filter characteristic may be defined for each element within the array. If these filter characteristics can be achieved, then approximately constant beam widths with relatively low side-lobes can be achieved over the desired operational frequency band so ensuring uniform coverage of the field of view. However, in practice, a filter design to achieve these characteristics could not be found. Although an approximation to the attenuation response could be achieved, the phase response could not be adequately controlled.